Six affiliated trade unions - including Unite and third largest GMB – did not mention the Labour Party at all anywhere on either the website’s front page, the “about us” section or membership page of the union’s website.

Peter Facey, the campaign group's director, said Labour had to stop “being so defensive” about the way it is funded by unions.

He said: “It is time Labour stopped being so defensive and looked at this as an opportunity not just to take big money out of politics but to actually strengthen its links with trade union members.

“The fundamental issue is ensuring that trade union members are properly informed about the exact relationship between their union and the Labour Party.

“It is clear from this research that most trade unions are less than explicit about affiliation to new recruits.”

According to Electral Commission documents, since Ed Miliband became Labour leader, 81 per cent of the party’s donations have come from unions.

Between the fourth quarter in 2010 and the second quarter 2012, Labour received a total of £21million. Of that figure £17millkion came from unions.

Mr Facey praised Unison for providing a “model example” of how a union could be upfront with members about their support for Labour.

He added: “If trade unions want to defend their links with Labour, it is high time they stopped being so shy about it in practice.”

A Labour party spokesman said: “We are pleased that millions of working people choose to support the Labour party.”

Party sources said that the reason may be because some unions have a political fund which is used for campaigning work, and not just supporting Labour.

At the party’s annual conference in Manchester, Labour members voted in favour of a motion that raised concerns about the “prolonged pay freeze” in the public sector. The views will now be fed into the party’s policy review.

Public sector unions are currently preparing for a major Spring campaign against pay freezes in the public sector.

They are furious with the Labour leadership for not fighting harder on the issue of public sector pay.

Last night a senior union leader tonight released a series of "policy errors" made by Ed Balls when he was in government.

Paul Kenny, general secretary of the GMB, published extracts from speeches made by Mr Balls in recent years on areas including financial regulation, the Private Finance Initiative and Public Private Partnership.

Dave Prentis, general secretary of Unison, also said workers wanted hope from their leaders, not "economic lectures which simply justify a Tory agenda".

A spokesman for Unite said last night that every week the Conservatives pointed to Unite's relationship with Labour.

He added: “We have a political page [on our website] that talks about our relationship with Labour. Trade unions have to operate under some of the strictest legislation governing trade unions in Europe.”